Coal is an abundant fossil fuel which is a convenient source of energy once it has been moved from mine to mouth to the site of use. Apart from the expense of mining, coal has two principal disadvantages. The first is that it is difficult and expensive to transport by the usual commercial means such as railroads, barges and ships. Second, since many coals contain high quantities of organic and inorganic sulfur, sulfur oxides, principally sulfur dioxide are present in large amounts in the combustion by-products. These oxides, when combined with moisture in the atmosphere form sulfurous and sulfuric acid which are the most damaging components of acid rain.
Much effort has been expended to alleviate or to avoid these problems. A principal effort to deal with the transportation problem has been to form coal-water mixes at or near the mine mount and to transport the mixes through pipe lines to the use site. However, coal particles do not remain suspended in water. Accordingly, any stoppage of the flow of the mixes can quickly lead to clogging of the pipelines, pumps and appurtenent equipment. Additionally, the coal must be utilized promptly upon arrival at the use site before it settles in the storage facility.
Heretofore a variety of organic and inorganic stabilizers have been studied in efforts to extend the storage time of the coal-water mixes. Those that are useful for this purpose are often expensive or burn with the production of by-products which are toxic or interfere with the smooth operation of the boiler equipment.
To limit the exhaust of sulfur oxides into the atmosphere, principal attention has focused on the direct injection of dry sorbent into the boiler and collection and disposal of spent sorbent and fly ash. The sorbent which has heretofore proven most attractive is limestone or calcium carbonate. It may be injected with the fuel, with the combustion air, or downstream in the combustion gases. The sorbent reacts with the oxides of sulfur to form a sulfated sorbent which is normally separated by electrostatic precipitation.
Limestone has a number of art recognized disadvantages as a sorbent. The above identified, concurrently filed patent application describes and claims the use of portland cement as a sorbent, and the advantages of such use.